Upon encountering antigen, naive CD4+ T helper precursor (Thp) cells are differentiated into two distinct subsets, Type 1 T helper (Th1) and Type 2 T helper (Th2). These differentiated Th cells are defined both by their distinct functional abilities and by unique cytokine profiles. Specifically, Th1 cells produce interferon-gamma, interleukin (IL)-2, and tumor necrosis factor (TNF)-beta, which activate macrophages and are responsible for cell-mediated immunity and phagocyte-dependent protective responses. In contrast, Th2 cells are known to produce IL-4, IL-5, IL-6, IL-9, IL-10 and IL-13, which are responsible for strong antibody production, eosinophil activation, and inhibition of several macrophage functions, thus providing phagocyte-independent protective responses. Accordingly, Th1 and Th2 cells are associated with different immunopathological responses.
In addition, the development of each type of Th cell is mediated by a different cytokine pathway. Specifically, it has been shown that IL-4 promotes Th2 differentiation and simultaneously blocks Th1 development. In contrast, IL-12, IL-18 and IFN-gamma are the cytokines critical for the development of Th1 cells. Accordingly, the cytokines themselves form a positive and negative feedback system that drives Th polarization and keeps a balance between Th1 and Th2.
Th1 cells are involved in the pathogenesis of a variety of organ-specific autoimmune disorders, Crohn's disease, Helicobacter pylori-induced peptic ulcer, acute kidney allograft rejection, and unexplained recurrent abortions. In contrast, allergen-specific Th2 responses are responsible for atopic disorders in genetically susceptible individuals. Moreover, Th2 responses against still unknown antigens predominate in Omenn's syndrome, idiopathic pulmonary fibrosis, and progressive systemic sclerosis.
There remains a high unmet medical need to develop new therapeutic treatments that are useful in treating the various conditions associated with imbalanced Th1/Th2 cellular differentiation. For many of these conditions the currently available treatment options are inadequate. Accordingly, the Th1/Th2 paradigm provides the rationale for the development of strategies for the therapy of allergic and autoimmune disorders.